1. Field of the Invention
The present invention relates to digital video cameras. More particularly, the invention relates to a dual sensor video camera including a panchromatic sensor and a color filter array (CFA) sensor.
2. Description of the Related Art
Digital video cameras utilize sensors that capture light from a scene and produce digital information representing images of the scene. The sensors have a large number of sensor sites that each capture light from a particular point within the scene, which is represented as a corresponding pixel in the digital image.
Two basic types of digital video cameras are known in the art: single sensor cameras and 3-sensor cameras. The sensor in a single sensor camera is typically overlaid with an alternating pattern (e.g., Bayer pattern) of color filters referred to as a color filter array (CFA). The color filter array typically comprises a pattern of red color filters, green color filters, and blue color filters, where each color filter is aligned over one of the sensor sites. Thus, the color filter over each sensor site filters either the red, green, or blue component of the light falling onto it so that each sensor site effectively captures either red, green, or blue color information.
A process called demosaicing or CFA interpolation is used to estimate the missing color components for each pixel in the image. For example, if a particular sensor site is overlaid with a red color filter so that it captures the red color component then the demosaicing algorithm estimates the green and blue color components for the corresponding pixel based on the green and blue color components measured by surrounding sensor sites that are overlaid with green and blue color filters.
The demosaicing process works quite well for many images. However, in some images, a problem called color aliasing occurs. For example, in an image with a lot of high frequency information (fine detail), the color information can change as fast as every pixel. In this situation the demosaicing algorithm has difficulty making appropriate estimates as to the missing color components for each pixel, with the result that spurious colors appear in the image.
One approach to this problem has been to overlay a low-pass filter over the sensor. A low-pass filter limits how quickly the image information can change. This solves the problem of color aliasing but destroys fine detail and makes all images fuzzier, whether they suffered from visible color aliasing or not.
The other type of video camera, the 3-sensor camera, uses a beam splitter to split the light into three light beams that are sent to three different sensors. One sensor is overlaid with a red color filter, one is overlaid with a green color filter, and one is overlaid with a blue color filter. The red, green, and blue color components of the image pixels are obtained from the corresponding sensor sites of the respective sensors.
This approach provides a very high quality result with no color aliasing. However, one problem with 3-sensor cameras is the cost and complexity involved in their production. For example, the three sensors are typically precisely mechanically aligned with each other so that their respective sensor sites correspond to the same pixels in the image, which adds manufacturing cost. Also, since the beam splitter splits light into three separate beams, the amount of light that reaches each sensor is reduced. The reduction in light to the sensors results in a lowered signal-to-noise ratio and effectively adds dynamic noise to the image.
Both single sensor and 3-sensor video cameras typically convert the image from RGB 4:4:4 format to an industry standard YCbCr 4:2:2 format.